Chemical mechanical polishing (CMP), also called chemical mechanical planarization, is a well-known process for removing oxide and other deposits from the surface of a wafer. CMP systems are frequently used during the processing of silicon semiconductor wafers. CMP systems are made by a number of vendors, including Applied Materials, Inc., of Santa Clara, Calif. Many conventional CMP systems polish semiconductor wafers by abrading the surface of the wafer with a silica-based slurry.
FIG. 1 illustrates selected portions of chemical mechanical polishing (CMP) system 100 according to an exemplary embodiment of the prior art. CMP system 100 comprises support platform 101, platen 105, polishing pad 110, pad conditioning disk 115, spindle 120, disk actuator 125, motor 130, and drive shaft 135. CMP system 100 further comprises motor 130, drive shaft 145, polishing head 150, motor 160, drive shaft 165, and slurry dispenser 170. Applied Materials (AMAT) manufactures the AMAT Mirra™ CMP system, which houses three CMP systems similar to CMP system 100 in an enclosure. It is noted that the components of CMP system 100 depicted in FIG. 1 are not drawn to scale. Rather, the sizes and relative positions of the components of CMP system 100 are selected for easy reference and explanation.
The operation of CMP system 100 is widely understood. Drive motor 140 and drive shaft 145 rotate platen 105 and polishing pad 110. Slurry dispenser 170 dispenses onto polishing pad 110 a silica-based slurry made from de-ionized water mixed with SiO2 (or KOH). Rotation of pad 110 carries the slurry underneath polishing head 150. A silicon wafer (not shown) is attached to the bottom surface of polishing head 150, which may be, for example, a Titan™ polishing head from Advanced Material, Inc. The wafer may be held in place on the bottom surface of polishing head 150 by vacuum pressure created by a membrane.
Motor 160 and drive shaft 165 rotate polishing head 150 and the attached wafer and press polishing head 150 and attached wafer downward onto polishing pad 110. This downward pressure forces the exposed surface of the attached silicon wafer into firm contact with the moving slurry dispensed on rotating polishing pad 110. The movement and pressure of the slurry abrades the exposed surface of the silicon wafer. The abrasion removes silicon oxide or other materials that are deposited on the exposed surface of the silicon wafer attached to the bottom of polishing head 150.
The efficient operation of CMP system 100 requires that the surface of polishing pad 110 be continually conditioned by pad conditioning disk 115. Polishing pad 110 may be made of polyurethane, for example. The surface of polishing pad 110 is covered by tiny grooves (e.g., depth=0.03 inch) that capture slurry particles. Pad conditioning maintains an acceptable oxide removal rate and stable performance. Pad conditioning helps maintain optimal pad roughness and porosity, thereby ensuring the even transport of slurry to the wafer surface. Without conditioning by pad conditioning disk 115, the surface of polishing pad 110 glazes and oxide removal rates decline.
The bottom surface of disk 115 is coated by an abrasive layer, such as a layer of nickel in which fine diamonds are embedded. Diamond pad conditioning disks are the most widely used method of pad conditioning in wafer fabrication facilities today. Pad conditioning disk 115 refreshes (or wears) the surface of polishing pad 110 during CMP processing to thereby maintain a uniform surface on polishing pad 110.
Disk actuator 125, motor 130 and drive shaft 135 drive pad conditioning disk 115, which is rigidly attached to spindle 120. Disk actuator 125 and drive shaft 135 contain the necessary gearing and other drive mechanisms to rotate spindle 120, thereby rotating disk 115. Disk actuator 125 and drive shaft 135 also contain the necessary drive mechanisms to sweep rotating disk 115 back and forth across the surface of rotating polishing pad 110.
The performance of pad conditioning disk 115 has a significant impact on the cost of operating CMP system 100. Aggressive use of pad conditioning disk 115 gives good process performance, but rapidly wears out polishing pad 110, thereby reducing pad life and increasing cost. A less aggressive use of pad conditioning disk 115 may not provide enough conditioning to polishing pad 110, resulting in unstable process performance.
Disk flatness is an important aspect of pad conditioning disk 115, since even wear across polishing pad 110 increases pad life and process stability. To ensure disk flatness, a new pad conditioning disk 115 must be broken in prior to use in an actual on-line CMP process. The process of breaking in a new disk 115 typically involves taking. CMP system 100 off line, removing the wafer and polishing head 150, and attaching new disk 115 to spindle 120. Next, new disk 115 scours the surface of pad 110 for approximately 30 minutes, until the bottom surface of new disk 115 is itself evenly worn.
At this point, broken-in disk 115 is removed, pad 110 is replaced with a new pad, polishing head 150 is re-attached, and CMP system 100 is re-qualified. The process of re-qualifying CMP system 100 may require another two hours. The AMAT Mirra™ CMP system, which houses three CMP systems similar to CMP system 100 in a single enclosure, may break in three pad conditioning disks 115 at a time. Nonetheless, the process of breaking-in pad conditioning disk 115 may take CMP system 100 off line for two and a half hours.
It is important to improve process performance by increasing productivity and reducing cost of ownership. However, taking CMP system 100 off line to break in new disks 115 makes achieving these goals more difficult. Reducing off-line time has the added benefit of minimizing the frequency of tool re-qualification, resulting in higher availability and more finished wafers per month.
Therefore, there is a need in the art for an improved chemical mechanical polishing (CMP) system that has reduced off line time. In particular, there is a need for an improved system and method for breaking in pad conditioning disks that reduce the amount of time that a chemical mechanical polishing (CMP) system must be taken off line.